240 HENRY C. TRACY 



SUMMARY 



1. The swimbladder in Clupeoids is composed of a mucosa 

 (tunica interna), a submucosa, and an outer dense connective- 

 tissue layer (tunica externa). 



a. In the body of the swimbladder and in the membranous 

 vesicles, the lining is simple squamous epithelium, but in the 

 pneumatic duct and in the precoelomic diverticulum the epi- 

 thelium is columnar and apparently glandular in structure. 



b. A circular layer of smooth muscle is found around the 

 pneumatic duct and at the beginning of the precoelomic 

 diverticulum. 



c. The glandular cells and the smooth muscle probably regu- 

 late the gas pressure in the swimbladder. 



2. The perimeningeal tissue is differentiated in certain places 

 into a compact elastic tissue (perilabyrinthine) which extends 

 away from the walls of the labyrinth under the brain (subcere- 

 bral plate) and in a band over the medulla just behind the cere- 

 bellum (supracerebral band). Less extensive developments of 

 this tissue are found in relation to other parts of the labyrinth. 



3. A complicated system of intercommunicating canals and 

 spaces of rarefied connective tissue extends through the peri- 

 labyrinthine tissue around the brain and labyrinth. These 

 spaces may be grouped in three divisions: 



a. Lateral, spaces in the tissue in the lateral recess near the 

 lateral-line canal and lateral to the labyrinth. 



b. Subcerebral, two canals traversing the under surface of 

 the subcerebral perilabyrinthine plate; the posterior of these 

 canals (saccular subcerebral canal) connects the saccular por- 

 tions of the auditory recesses of the two sides; the anterior canal 

 (utricular subcerebral canal) connects the lateral recesses of the 

 two sides of the skull by passing under the utriculus where it is 

 partially subdivided into two canals by the projection of the 

 lips of the fenestra of the anterior osseous capsule. 



c. Supracerebral, the canal in the supracerebral perilabyrin- 

 thine band over the medulla. 



